1. Field of the Invention
The present invention relates to products having compositions incorporating rubber crumbs derived by recycling rubber tires and, more particularly, to the manufacture of slabs of construction material containing recycled tire rubber, and products thereof.
2. The Prior Art
An ongoing critical need exists for an environmentally compatible construction material that is wear, weather and chemical resistant. The present invention contemplates the use, as a substantial ingredient in such construction material, of vulcanized rubber crumbs derived from the ever increasing supply of scrap rubber tires. The present invention is environmentally compatible in the sense that it utilizes a growing accumulation of scrap that is very difficult to discard.
The problem of scrap tires and their disposal has occupied the attention of many for years. Some have suggested using whole scrap tires for erecting structures. See Martin Pawley, Building for Tomorrow: Putting Waste to Work, Sierra Club Books, San Francisco, 1982.
Others have developed machines designed to cut the tread portion from the tire for use as a roof covering. Ibid. Still others have turned to incineration for the direct production of energy, or to tire rubber chips as a fuel supplement in paper and pulp mills, cement kilns and other industrial heat generators. For one reason or another, few of these recycling efforts have been adequately successful commercially and environmentally.
Recycled rubber, as defined by the Rubber Recycling Division of the National Association of Recycling Industries, Inc., generally refers to any sort of rubber waste, including scrap tires, that has been converted into an economically useful form, such as reclaimed rubber, ground rubber, reprocessed rubber and die-cut punched rubber parts. Reclaimed rubber, on the other hand, represents a product resulting from a process in which waste vulcanized scrap rubber is treated to produce a plastic-like material that can be easily processed, compounded and re-vulcanized with or without the addition of either natural or synthetic binders. It is, of course, known that the vulcanization process technically, by definition, is irreversible. Nevertheless, an accepted definition for "devulcanization" is a change in vulcanized condition which results in decreased resistance to deformation at ordinary temperatures.
Most techniques for reclaiming scrap rubber involve batch processing. In one continuous technique, the scrap rubber is ground, and any foreign components such as metal and/or fiber are mechanically separated from the ground rubber remainder, which is then further ground to finer particle size. Then the finely ground rubber, together with various reclaiming agents, are metered into a blending system and conveyed to a special screw-extrusion machine. The screw-extrusion machine is jacketed along the screw's axial length to provide for several zones of controlled temperature, employing either hot oil or cooling water. The clearance between the screw and the jacketed chamber wall is close and adjustable. In the screw-extrusion machine, the finely ground rubber is subjected to a controlled and variable amount of high heat and pressure in a continuously moving environment. Softened rubber is then continuously discharged from the extrusion head, whereupon it is cooled and conveyed to the millroom for the final stage of the reclaiming process. For screw extruders, see D. H. Morton-Jones, Polymer Processing, Chapman and Hall, 1989.
In the millroom, the softened and now cooled rubber is generally mixed with ingredients in a suitable blender, then rewarmed and replasticated in a barrel-mixer. Thence it is fed first to a high-friction breaker mill and next to a high-friction refiner mill. A high-friction ratio for the refiner mill is achieved with different-sized rolls rotated at considerably different speeds. The rolls are set tightly to obtain a thin sheet of reclaimed rubber that is smooth, uniform and free of grain or lumps. The finished thin sheet of reclaimed rubber emanating from the refiner mill is pulled to a wind-up drum, and allowed to build up to a thickness of about an inch before being cut therefrom by a knife. The resulting reclaimed rubber slab is dusted to reduce tackiness and stacked. Since reclaimed rubber is cheaper than natural or synthetic rubber, it is widely used in the manufacture of new rubber goods, including new tires.
The present invention relates to a novel process of recycling, not reclaiming, scrap tire rubber, and to various recycled rubber products so made, not including tires.